Due date help

Ok, I am really confused.
We had a frozen embryo transfer on the 4th of March this year.
The embryo was 5 days old when it went in. And they put it in 5 days after ovulation.
The transfer was successful and we are pregnant But when am I due?
If I do the calculation based on LMP, I am due on 18th November.
However, I had a scan yesterday which put me at 7 weeks and 6 days (plus or minus 5 days), and the fertility clinic gave me a due date of 24th November. They gave me the gestational age based on size. Surely since they know when they put it in, they should know exactly how old it is???? Also, when they gave me the paperwork to go to the obstetrician that had a date of 22nd November. I can't get in to see the ob until June, so I'm still with the fertility clinic until then.
I'm so confused.
I don't suppose it's that much of a big deal, I mean baby will come when it's ready, but everyone I know who has been pregnant seems to know a specific date.
Also, what's with the plus or minus 5 days? I've been wanting to follow my pregnancy week by week, following it growing from a sesame seed to a jelly bean, to a grape. But know I don't know where I am! Lol.
Anyone else confused?

GymbaROO offers activities for babies & toddlers in a fun learning centre, focussing on developmental education. Classes are available Australia-wide. Enrol today & help your child to reach their full potential. Visit the website to find out more.